In the X-ray computed tomography, use is made of a multielement radiation detector consisting of a plurality of radiation detector elements that are arranged on an arc, in order to irradiate a material to be inspected with a fan-shaped beam of X-rays thereby to measure the distribution of X-ray absorption factors. As the above-mentioned multielement radiation detector, there has been known an ionization chamber type detector utilizing the ionization function of a gas, a semiconductor detector utilizing the ionization function of a solid material, and a scintillation detector utilizing the fluorescent function relying upon X-rays.
In the scintillation detector, use has generally been made of a single crystal inorganic scintillator, such as NaI, CsI, CdWO.sub.4, Bi.sub.4 Ge.sub.3 O.sub.12, or the like. These scintillators have their merits and demerits in their characteristics, and are not utilizable for the computed tomographies of all types. To compensate the single crystalline scintillator for the above-mentioned defect, there has been used a powder scintillator which is molded by using a suitable binder such as a polystyrene resin or the like.
The powder scintillator has a smaller transmission coefficient of light than the single crystal scintillator. Therefore, it is difficult to take out the rays from the outgoing side and to receive the rays using a light detector. Therefore, the structure adapted to the radiation detector employing powder scintillator consists of receiving, using a light detector, the light from the incident side of the powder scintillator. The detector of this structure has already been proposed in the U.S. Pat. No. 4, 187,427. In this detector, however, the scintillator is aslantly disposed with respect to the incident direction of X-rays, and the light detector is disposed at a position off the X-ray incident path in parallel with the incident direction of X-rays. Therefore, the sensitivity for detecting the X-rays has gradient relative to the thickness direction of the incident beam of X-rays.
In the X-ray computed tomography, on the other hand, difference in the sensitivity among the individual elements constituting the multielement detector gives rise to the occurrence of ring-like artifact in the picture. Therefore, electric calibration has been performed to substantially eliminate variance in sensitivity among the elements. The ring-like artifact can be prevented from occurring by the calibration only when the X-ray absorption factor of the material to be inspected is uniform in the thickness direction of the beam of X-rays. With the practical materials to be inspected, however, the X-ray absorption factor is not uniform in the thickness direction of the beam of X-rays. Furthermore, if the individual elements have gradient in sensitivity in the thickness direction of the beam of X-rays, it becomes difficult to effect the calibration to match the practical imaging operation. Variance in sensitivity develops among the individual elements due to the gradient in sensitivity. Therefore, ring-like artifact develops on the reproduced picture.